Influence Of Plasma Nitriding And Surface Hardening Composite Treatment On Contact Fatigue Properties Of 42CrMo Gear Steel

The method of surface modification of 42 CrMo gear steel by vacuum pulse nitriding technology has been widely applied in production practice.However,due to the unreasonable control of process parameters,uneven distribution and part of lost of nitrided layers always lead to some problems like low life and poor reality on gear and key components.Therefore,the optimum parameters of plasma nitriding and the optimization method of surface modification before nitriding are explored.And contact fatigue properties of 42 CrMo gear under different treating measures were investigated combing with microstructure and surface integrity.Reasonable plasma nitriding and composite treatment parameters are evaluated by the cross-sectional microstructure,hardness gradient distribution and surface residual stress of nitrided layer.The microstructure,morphology,nitrogen content,phase composition of the nitrided layer were examined and analyzed by the means of optical microscope,scanning electron microscopy,transmission electron microscopy and X-ray diffractometer.Additionally,the surface integrity of the nitrided layer was analyzed by the means of XRD residual stress tester,surface roughness tester and microhardness tester.Finally,the failure modes of contact fatigue fracture were analyzed by scanning electron microscopy.The relationship and law between the three aspects of microstructure,surface integrity and contact fatigue performance are established,which provide theoretical and practical basis for anti-fatigue manufacturing of gear.The following main conclusions are obtained:(1)The optimum parameters of vacuum pulse plasma nitriding for 42 CrMo steel is 200 Pa,530?,8h.The thickness of bright white and total nitrided layers arerespective 20?m and 387?m,which there are smooth transition of hardness gradient and maximum residual compressive stress of-803 MPa.The surface nitrided layer is FexN nitride composed by ? and ?'phases.Most of the nitrides are large particles and unevenly distributed in tissues.The subsurface nitrided layer is a solid solution,which most of nitrogen atoms are soluble in the BCC cubic of ?-Fe.There are some granular nitrides are precipitated and tend to assemble at the interface.(2)With the increasing of temperature from 450? to 560?,the microhardness of surface nitrided layer is gradually increased which the maximum is 882HV0.1 and the thickness is also increased from 150?m to 462?m.The change regulation of surface residual stress shows increases first and then decreases,which is related to the volume fraction of ? phase.There are highest nitrogen content and ? phase volume fraction when nitriding temperature is 530?.It further verifies the reasonable process parameters of plasma nitriding.(3)The reasonable surface pretreatment process of 42 CrMo steel is WSP parameters with 10 mm,500mm/min,100MPa×2.Compared with nitriding,the thickness of bright white and total nitrided layers are obviously increased by composite treatment.And the volume fraction of ? phase is also increased.Before WSP pretreatment,Nitrides forms strip distribution and readily enriched at the grain boundaries.After WSP pretreatment,Nitrides tends to irregular and uniform distribution.WSP pretreatment can cause a large number of high density dislocations in the plastic deformation of the material surface,which provide reasonable fast nitrogen diffusion channels.It is the reason that the effective hardening and residual stress influence layers both increase more than 100?m.(4)Under the same contact stress,the fatigue life of nitriding has been increased5.36 times compared with original.At the same time,the fatigue failure changes from pitting corrosion to shallow exfoliation.And the fatigue life of composite treatment is also increased 2.33 times compared with nitriding.The conditional fatigue limit of WSP pretreatment(856MPa)is higher than nitriding(699MPa).It is attributed to accelerated permeation and the complex distribution of nitride particles by the composite treatment of WSP pretreatment combined with nitriding.Besides,thesurface integrity is improved owing to increasing of the effective hardening and residual stress influence layers.They can suppress the propagation of fatigue subsurface cracks and the initiation of secondary cracks.Therefore,the contact fatigue property of 42 CrMo steel has been improved.